WASHINGTON — President Trump’s infrastructure plan got its first public airing in Congress on Thursday, with some of the thornier issues that are likely to guide debate about any forthcoming public works package on display.

These issues are mainly centered on funding and environmental permitting. Transportation Secretary Elaine Chao and R.D. James, assistant secretary of the Army for civil works, who leads the Army Corps of Engineers, appeared before the Senate Environment and Public Works Committee on behalf of the administration.

Chao brushed off a question about President Trump’s current position on raising the federal gas tax. In recent weeks, Trump has floated the idea of a 25-cent gas tax increase to help fund infrastructure.

Sen. Chris Van Hollen, a Maryland Democrat, asked about this. “The president has now said on a number of occasions that he does support an increase in the gas tax to fund this $200 billion plan,” he said. “Does the president mean what he says about increasing the gas tax?”

“You should ask the White House,” Chao shot back.

Later in the day, DJ Gribbin, a special assistant to the president on infrastructure policy, said, as he has previously, that the president has not ruled out the possibility of a gas tax increase. “He’s said supportive things about it,” Gribbin told state transportation officials at an event.

“As an administration we don’t have a position for or against,” he added.

The president’s infrastructure plan calls for $200 billion of federal spending that would mostly go to new grant programs. It aims to stimulate around $1.5 trillion of spending over a decade for roads, water systems and other public works, when factoring in state, local and private funds.

How lawmakers will find or raise the money to pay for new infrastructure spending, or even to shore-up existing programs in future years, like the Highway Trust Fund, is one of the quandaries on Capitol Hill as infrastructure discussions ramp up.

Gribbin has previously pointed to proposed cuts in the president’s fiscal year 2019 budget plan as a way to pay for the infrastructure programs the White House proposed. Democrats have lashed out against the idea of chopping existing funding for transit and rail.

Sen. Jeff Merkley, an Oregon Democrat, questioned whether the White House infrastructure and budget proposals amount to “simply moving chairs around on the deck of our infrastructure Titanic.”

And Sen. Tom Carper, a Delaware lawmaker who is the top Democrat on the Environment and Public Works Committee, said he and other senators met with governors who are “concerned” and are “not anxious to accept the kind of deal” outlined in the Trump proposal.

The largest pool of grants under the White House public works plan involves an “incentives program” that would be allotted $100 billion.

This initiative would be designed to favor states and local governments that can bring greater shares of non-federal dollars to the table for projects. And grants through the program would not be allowed to exceed 20 percent of new project revenue.